Method and apparatus for determining colorimetric concentrations

ABSTRACT

A method and an apparatus for determining colorimetric concentrations wherein the difference between the light absorption of a medium in two or more different wave-length ranges is measured by means of an automatic wave-length switching, whereupon the concentration of the medium is calculated in dependence of said difference and the absolute absorption of the medium in one of the wave-length ranges.

United States Patent Lindberg [451 Dec. 19, 1972 [54] METHOD ANDAPPARATUS FOR 3,250,174 5/1966 Lutz ..356/188 x DETERMINING COLORIMETRICFOREIGN PATENTS OR APPLICATIONS CONCENTRATIONS [72] Inventor: Ame RobertLindberg Slingergatan 1,176,395 8/1964 Germany ..250/226 1 4 l 8 3 39Molndal Sweden Primary Exammer--Walter Stolwem Filedi 1970Attorney-Albert M. Parker, Charles G. Mueller, Larimer P. Brooks, AlfredL. Haffner, Jr., Harold [21] 87079 Haidt and G. Thomas Delahunty [52]US. Cl. ..356/188, 250/218, 250/226 [57] ABSTRACT [51] Int. Cl ..G01j3/48 A method and an apparatus for determmmg [58] Field of Search..250/218, 226, 210, colorimetric concentrations wherein the difference356/179'484 189 between the light absorption of a medium in two or moredifferent wave-length ranges is measured by [5 6] References cued meansof an automatic wave-length switching, whereu- UNITED STATES PATENT ponthe concentration of the medium is calculated in S dependence of saiddifference and the absolute ab- 3,171,027 2/1965 Wallack ..356/189Xsorption of the medium in one of the wave-length 1,898,219 2/1933 Sharp..356/188 range, 3,529,895 9/1970 Pincus ..356/188 X 2,872,588 2/1959Barton, Jr ..250/210 8 Claims, 11 Drawing Figures PATENTEU 05c 19 I972SHEET 1 BF 2 ml m Q x QNFIII PATENTED DEC 19 I972 SHEET 2 [IF 2 METHODAND APPARATUS FOR DETERMINING COLORIMETRIC CONCENTRATIONS The presentinvention relates to a method and means for determining colorimetricconcentrations, for example of chemicals in a solution which, with orwithout reagent, is ofa color the intensity of which is a function ofits concentration.

As is known, the basic color and turbidity of the medium or the reagentis a cause of incorrect determination in all cases of determiningcolorimetric concentrations. In manual colorimetric analysis thesesources of error can often be counteracted by, inter alia, specialcalibration and various types of compensation. In an automatic analysisof process media, this is not usually possible.

It has now been found possible due to the present invention to achievesuch compensation automatically, as well as compensation for ageing ofthe light source and the electronic apparatus. Substantiallycharacteristic of the method proposed according to the invention is thatby means of automatic wave-length switching, differences are measuredbetween the light absorption of a medium in two or more different rangesof wave-length, after which the concentration of the medium iscalculated depending on the difference measured and the absoluteabsorption of the medium in one of the ranges of wave-length.

According to a suitable embodiment of the invention the division of thewave-lengths in the flow of light is effected by means of a rotatingfilter disc divided into at least two different filter sectors, in whichcase the absorbing medium only affects that section ofv the wavelengthrange covering one filter sector and the background absorption remainsthe same in both the filter sectors.

In order to carry out the method proposed according to the invention adevice may suitably be used comprising a light source, a measuring bulbcontaining the medium for measuring the concentration, and aphotoelectric element registering the light flow through the medium andarranged 'to cooperate with an electronic output device and beingcharacterized in that a wavelength switching means located between thephotoelectric element and the measuring bulb is arranged to generatedifferent wave-length ranges alternately in the light fiow for recordingby the element and subsequent evaluation in a differencemeasuringbridge.

According to a suitable embodiment'of the invention the wave-lengthswitching means suitably consists of a rotating filter disc having atleast two different filter sectors.

In the following the invention will be described further with referenceto the accompanying drawings in which FIG. 1 shows a diagrammatic sketchof a device according to the invention for colorimetric determination,

FIG. 2 a view of the rotating filter disc which is a part of the deviceaccording to FIG. 1,

FIG. 3 a detailed view of the sector disc in the device according toFIG. 1, and

FIGS. 4 11 various diagrams showing the principal function of the filterchanging system used according to the invention.

In the drawings 1 designates a light source which, through a first lenssystem 2, directs the flow of light through a measuring bulb 3containing a medium to be examined. After passing through the measuringbulb 3, the light is broken up by a second lens system 4 and focused ona photo-electric element 6. The signal from the element 6 is amplifiedand linearized, possibly with respect to light absorption in relation tothe concentration, in an amplifier 7. Between the second lens system 4and the photo-electric element 6 is a rotating filter disc 8. The filterdisc 8 has at least two different filter sectors F,, F and cooperateswith a stationary compensation filter 5 arranged between the filter disc8 and the element 6 to adjust the total light level and for finaladjustment of the balance in the filter system. When the filter disc 8rotates different ranges of wavelength of the flow of light will beallowed to pass to the photoelectric element 6 and from this a pulsatingdirect current is obtained which corresponds to the light absorption ofthe medium at these wave-lengths. The rotation of the filter disc 8 iseffected by a motor 9 and, besides the filter disc 8 on the drivingshaft of the motor 9, there is also a sector disc 10. A sensing device11 is arranged near the sector disc 10 and this, via asynchronizingelectronic device 12 over the relays R and R sees to itthat the signal from the'photo-electric element 6is fed to one or theother half of, the measuring bridge 13, depending on the position of thefilter sectors F, or F in relation to the flow of light. The voltagedifference between the capacitors C, and C in the measuring bridge 13 ismeasured and amplified in a final amplifier l6 and can then be fed to anindicating instrument 14 or a recorder 15. The measuring equipment isadjusted to zero by means of an adjusting resistor R and the measuringrange in question is set by a resistor R,.

The function of the device is clear from FIGS. 5 l 1. Here a systemaccording to the invention having two filter sectors F, and F in therotating filter disc 8 and a stationary filter (compensating filter 5)which upwardly limits the transmission range prevailing from the pointofview of wave-lengths, for the two rotating filter sectors. In FIGS. 5 11the compensation filter has been designated F,,.

A grey filter FG is also added to the filter sectors F,. This isadjusted so that the flow of light, represented by the areas A, and B,in FIGS. 4 and 5, through the two filter sectors F,, F is equal.However, as is clear from FIG. 4, about- 40 percent of the transmissionis screened off by the grey filter FG, but a wider range of wave-lengths(ca 500 800 my.) is then covered by the filter sector F (see FIG. 5 inthe wave-length range of 600 750 mp).

FIGS. 6 and 7 show a transmission curve for a medium having a backgroundabsorption of about 20 percent calculated on the light intensity. Aswell as the absorption in question from' the measuring point of viewwhich corresponds to'the area F,, the measurements in the differentranges of wave-length are affected by an absorption corresponding to thesurfaces 0,, G (see FIGS. 8 and 9). The background absorption of themedium, shown in FIG. 6, of 20 percent of the light intensity will be,as seen in FIG. 8, 20 percent calculated on the remaining lightintensity percent) after the grey filter, i.e. 12 percent, whereastheabsorption in FIGS. 5 and 7 will remain the same, Le. 20 percent ofthe light intensity. Since the filter sectors F,, F, cover wave-lengthranges of equal size matched so that the light flows, i.e. the areas A,and B,, are the same size at the start, the light flows, i.e. the areasA, and B will also be equal in size, and thus also the areas G, and GThe signal level corresponding to the difference between the areas A,and A or B, and B is changed. On the other hand the difference of thesignal level corresponding to the areas A and B will be unchanged uponvarying background absorption. The difference in the light flow to thephoto-electric element 6 measured at two different wave-length ranges,as shown above, is thus irrespective of the background absorption. Whenan absorbing medium, as shown in FIGS. 6 and 7,

represented by the area F,,, only influences the part of the wave-lengthrange covering one of the filter sectors, in this case F,, but not thewave-length range of the other filter sector F a difference arisesbetween the areas A and B, as can be seen in FIGS. 10 and 1 1. Thisdifference A, B taking into account the difference between A, A can beconverted to a concentration of the medium in question. The differenceA, A which represents the alteration of the absolute signal level, cancontrol a logarithmic conversion element or automatically alter theworking point with the use of a logarithmic photo-electric element. Thedifference between A and B, can thus easily be converted to aconcentration.

The invention described above can be used generally in all fields inwhich it is suitable by filters or some other division of thewave-lengths to measure the difference between two ranges ofwave-lengths and has shown particularly good results in automaticallydetermining chemical oxygen consumption with bichromate oxidation,determining the phosphorous content in waste water and chemical oxygenand consumption with potassium permanganate.

However, the invention is not limited to the embodiment shown in thedrawings, but can be varied in many ways within the scope of thefollowing claims. Thus the filter sectors may be of different sizes andthe balancing of the energy to the photo-electric element may be done bygrey filters, for example. Conversely, it is not always necessary to usegrey filters as the system can be balanced by filter sectors ofdifferent sizes. The compensation filter described above may also haveother functions and be, for example, a color filter which for specialpurposes adds effects to the filter sectors.

What 1 claim is:

l. A method of determining colorimetric concentrations in liquidscomprising passing a beam of light through liquid in a measuring bulb,sequentially trimming the light transmitted through the liquid bysequentially intercepting said transmitted light with at least twoabsorption filter sectors of a rotating filter disc, one of saidabsorption filter sectors transmitting light in a wide range ofwave-lengths, and another of said absorption filter sectors transmittinglight only in a small range of wave-lengths within said wide range, thewave-lengths absorbed by the concentration being determined in theliquid being within the wide range but not being within the small range,balancing the flow of li ht energy through said filter sectors, interceting the ight passed through said filter sectors with a p otoelectriccell for producing electrical signals corresponding to the light passedthrough the different filter sectors, and electronically comparingsaidsignals for determining the colorimetric concentration from thedifference between said signals.

2. The method of claim 1 including employing only two different filtersectors, alternately passing light through each said filter sector,synchronizing the rotation of the filter disc with relay means forsegregating the electrical signals corresponding to each filter sector,and evaluating the difference between said signals with a differencemeasuring bridge.

3. Apparatus for determining colorimetric concentrations in liquidscomprising a single light source, a measuring bulb for liquid to beexamined and a photoelectric cell positioned to register light flow fromsaid source transmitted through liquid in said bulb, a filter dischaving at least two absorption filter sectors mounted for rotation toplace said filter sectors sequentially in the path of light passing fromsaid bulb to said cell, one filter sector of said filter disc passinglight in a wide wave-length range including the wave-lengths absorbed bythe concentration to be determined, another filter sector of said filterdisc passing light only in a narrow wave-length range within said widerange, said'narrow wave-length range not including the wave-lengthsabsorbed by the concentration to'be determined, a difference measuringbridge operatively connected with said photo-electric cell for measuringdifferences in electrical signals produced by said cell, means forrotating said filter disc and means for synchronizing the flow ofsignals from said photo-electric cell to different sides of said bridgewith the rotation of said disc whereby the difference measured by saidbridge represents the difference between light flows through said filtersectors.

4. The apparatus of claim 3 wherein the filter sectors of said filterdisc are of equal size.

5. The apparatus of claim 3 wherein the filter sectors are of differentsizes and including a compensation filter for balancing light energyflowing to the photoelectric cell.

6. The apparatus of claim 3 and including a compensation filterpositioned between the filter disc and the photo-electric cell.

7. The apparatus of claim 3 and including a grey filter added to onefilter sector for adjusting the flow of light therethrough so that flowof light through the filter sectors is equal.

8. The apparatus of claim 3 wherein the synchronizing means comprises asector disc mounted on the same axis with the filter disc for concurrentrotation therewith and a sensing device positioned near the sector discfor controlling the flow of signals to said different sides of thebridge by means of relays.

1. A method of determining colorimetric concentrations in liquidscomprising passing a beam of light through liquid in a measuring bulb,sequentially trimming the light transmitted through the liquid bysequentially intercepting said transmitted light with at least twoabsorption filter sectors of a rotating filter disc, one of saidabsorption filter sectors transmitting light in a wide range ofwave-lengths, and another of said absorption filter sectors transmittinglight only in a small range of wave-lengths within said wide range, thewave-lengths absorbed by the concentration being determined in theliquid being within the wide range but not being within the small range,balancing the flow of light energy through said filter sectors,intercepting the light passed through said filter sectors with aphoto-electric cell for producing electrical signals corresponding tothe light passed through the different filter sectors, andelectronically comparing said signals for determining the colorimetricconcentration from the difference between said signals.
 2. The method ofclaim 1 including employing only two different filter sectors,alternately passing light through each said filter sector, synchronizingthe rotation of the filter disc with relay means for segregating theelectrical signals corresponding to each filter sector, and evaluatingthe difference between said signals with a difference measuring bridge.3. Apparatus for determining colorimetric concentrations in liquidscomprising a single light source, a measuring bulb for liquid to beexamined and a photo-electric cell positioned to register light flowfrom said source transmitted through liquid in said bulb, a filter dischaving at least two absorption filter sectors mounted for rotation toplace said filter sectors sequentially in the path of light passing fromsaid bulb to said cell, one filter sector of said filter disc passinglight in a wide wave-length range including the wave-lengths absorbed bythe concentration to be determined, another filter sector of said filterdisc passing light only in a narrow wave-length range within said widerange, said narrow wave-length range not including the wave-lengthsabsorbed by the concentration to be determined, a difference measuringbridge operatively connected with said photo-electric cell for measuringdifferences in electrical signals produced by said cell, means forrotating said filter disc and means for synchronizing the flow ofsignals from said photo-electric cell to different sides of said bridgewith the rotation of said disc whereby the difference measured by saidbridge represents the difference between light flows through said filtersectors.
 4. The apparatus of claim 3 wherein the filter sectors of saidfilter disc are of equal size.
 5. The apparatus of claim 3 wherein thefilter sectors are of different sizes and including a compensationfilter for balancing light energy flowing to the photoelectric cell. 6.The apparatus of claim 3 and including a compensation filter positionedbetween the filter disc and the photo-electric cell.
 7. The apparatus ofclaim 3 and including a grey filter added to one filter sector foradjusting the flow of light therethrough so that flow of light throughthe filter sectors is equal.
 8. The apparatus of claim 3 wherein thesynchronizing means comprises a sector disc mounted on the same axiswith the filter disc for concurrent rotation therewith and a sensingdevice positioned near the sector disc for controlling the flow ofsignals to said different sides of the bridge by means of relays.